Carbon fiber is attracting increasing attention in the fields of storage batteries (lithium-ion batteries, electric double layer capacitors, etc.) and fuel cells. Especially, nonwoven fabric made of carbon fiber is gaining much attention as an electrode material and a conductive assistant for such batteries/cells. The nonwoven fabric is formed of carbon fiber whose fiber diameter is approximately 10 μm.
In recent years, nonwoven fabric formed of carbon fiber whose fiber diameter is 10 μm or less (e.g., approximately 1 μm) is being requested from the viewpoint of increasing the surface area.
Meanwhile, a technology performing carbonization and graphitization after performing electrostatic spinning by melting pitch has been proposed (Patent Literature 1).
The carbon fiber obtained by this method is low in the fiber diameter variations and is also high in the degree of graphitization. However, since only carbon sources of high carbonization ratio are used in this method, the degree of shrinkage of the material is low in the carbonization and the graphitization. Consequently, the aforementioned carbon fiber whose fiber diameter is 10 μm or less is hardly obtained. Further, only soft pitch (melting point: 300° C. or lower) is used in the technology of the Patent Literature 1, that is, the use of mesophase pitch or hard pitch (melting point: 300° C. or higher) has been impossible. Furthermore, since application of voltage to the pitch while heating the pitch is necessary, the apparatus is necessitated to be complex and the productivity is low.
As a method for producing such carbon fiber whose fiber diameter is 10 μm or less (e.g., approximately 1 μm), there has been proposed a technology performing a carbonization process and a shredding process after performing electrostatic spinning by using resin containing carbon sources (Patent Literature 2). According to this method, the use of catalytic metal is unnecessary and the electrical conductivity of the obtained carbon fiber is relatively high. Nevertheless, low electric resistance is required for use as battery materials and further improvement in the electrical conductivity is being requested in these years. In addition, due to the employment of the electrostatic spinning method, the productivity of the technology of the Patent Literature 2 is low, leading to high costs.